Optical emission studies of nitrogen plasma generated by IR CO2 laser pulses

摘要

Large-scale plasma produced in nitrogen gas at room temperature and pressures ranging from 4 x 10(3) to 1.2 x 10(5) Pa by high-power laser-induced dielectric breakdown (LIDB) has been investigated. Time-integrated optical nitrogen gas spectra excited from a CO2 laser have been measured and analysed. The spectrum of the generated plasma is dominated by the emission of strong N+ and N and very weak N2+ atomic lines and molecular features of N-2(+)(B-2 Sigma(+)(u)-X-2 Sigma(+)(g)), N-2(+)(D-2 Pi(g)-A(2) Pi(u)), N-2(C-3 Pi(u)-B-3 Pi(g)) and very weak N-2(B-3 Pi(g)-A(3) Sigma(+)(u)). The relative intensities of the 0-0 band heads in the N-2(C-B) and N-2(+)(B-X) systems are very weak as compared with the chemiluminescence spectrum of nitrogen formed in a glow discharge. An excitation temperature T-exc = 21000 +/- 1300 K was calculated by means of the relative intensity of ionized nitrogen atomic lines assuming local thermodynamic equilibrium. Optical breakdown threshold intensities in N-2 at 9.621 mu m have been determined. The physical processes leading to the LIDB of nitrogen in the power density range 0.4 < J < 4.5 GW cm(-2) have been analysed. From our experimental observations we can suggest that, although the first electrons must appear via multiphoton ionization or natural ionization, electron cascade is the main mechanism responsible for the LIDB in nitrogen.